Resistive random-access memory devices with multi-component electrodes

ABSTRACT

The present disclosure relates to resistive random-access memory (RRAM) devices. In some embodiments, a RRAM device may include a first electrode, a second electrode, and a switching oxide layer positioned between the first electrode and the second electrode, wherein the switching oxide layer comprises at least one transition metal oxide. The second electrode may include a first layer comprising a first metallic material and a second layer comprising a second metallic material. In some embodiments, the first metallic material and the second metallic material may include titanium and tantalum, respectively. In some embodiments, the second electrode may include an alloy of tantalum. The alloy of tantalum may contain one or more of hafnium, molybdenum, niobium, tungsten, and/or zirconium. In some embodiments, the alloy of tantalum contains a plurality of alloys of tantalum.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to U.S. patent application Ser. No. ______ entitled “Resistive Random-Access Memory Devices with Multi-Component Electrodes” (attorney docket number 200520.2-US2), filed on the same day as the present application, which is assigned to the assignee of the present application and is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The implementations of the disclosure relate generally to resistive random-access memory (RRAM) devices and, more specifically, to RRAM devices with multi-component electrodes.

BACKGROUND

A resistive random-access memory (RRAM) device is a two-terminal passive device with tunable and non-volatile resistance. The resistance of the RRAM device may be electrically switched between a high-resistance state (HRS) and a low-resistance state (LRS) by applying suitable programming signals to the RRAM device. RRAM devices may be used to form crossbar arrays that may be used to implement in-memory computing applications, non-volatile solid-state memory, image processing applications, neural networks, etc.

SUMMARY

The following is a simplified summary of the disclosure in order to provide a basic understanding of some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is intended to neither identify key or critical elements of the disclosure, nor delineate any scope of the particular implementations of the disclosure or any scope of the claims. Its sole purpose is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description that is presented later.

According to one or more aspects of the present disclosure, a resistive random-access memory (RRAM) device may include: a first electrode; a second electrode; and a switching oxide layer positioned between the first electrode and the second electrode. The switching oxide layer includes at least one transition metal oxide. The second electrode includes: a first layer including a first metallic material; and a second layer including a second metallic material. The second layer including the second metallic material is fabricated on the first layer including the first metallic material.

In some embodiments, the RRAM device further includes a substrate, wherein the first electrode is fabricated on the substrate, and wherein the switching oxide layer is fabricated on the first electrode.

In some embodiments, the first metallic material includes titanium (Ti).

In some embodiments, the second metallic material includes tantalum (Ta).

In some embodiments, a thickness of the first layer including the first metallic material is between 0.2 nm and 5 nm.

In some embodiments, the second layer including the second metallic material is thicker than the first layer including the first metallic material.

In some embodiments, a thickness of the second layer including the second metallic material is between 5 nm and 300 nm.

In some embodiments, at least one transition metal oxide includes at least one of HfO_(x) or TaO_(x), wherein x may be used to indicate the oxide being oxygen deficient compared to its full (or terminal) oxide and the value of x may be varied from the oxygen to metal atomic ratio in the stoichiometry of its full oxide, such as x≤2.0 for HfOx (where HfO₂ being the full oxide), and x≤2.5 for TaOx (where Ta₂O₅ being the full oxide).

In some embodiments, the first electrode includes platinum (Pt) or palladium (Pd).

In some embodiments, a dimension of the RRAM device is not greater than 1 micrometer (μm).

In some embodiments, a dimension of the RRAM device is between 1 μm and 1 nanometer (nm). In some embodiments, the dimension of the RRAM device is between 1 μm and 2 nm. In some embodiments, the dimension of the RRAM device is between 1 μm and 5 nm. In some embodiments, the dimension of the RRAM device is between 1 nm and 10 nm. In some embodiments, the dimension of the RRAM device is not greater than 10 nm. The dimension of the RRAM device is a lateral dimension or a critical dimension of the RRAM device.

In some embodiments, a surface of the first layer including the first metallic material directly contact a surface of the second layer including the second metallic material.

In some embodiments, the second layer including the second metallic material includes an alloy of tantalum.

In some embodiments, the alloy of tantalum further comprises at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.

In some embodiments, the alloy of tantalum comprises at least one of a binary alloy containing tantalum, a ternary alloy containing tantalum, a quaternary alloy containing tantalum, a quinary alloy containing tantalum, a senary alloy containing tantalum, or a high order alloy containing tantalum.

According to one or more aspects of the present disclosure, methods for fabricating an RRAM device are provided. The methods include fabricating, on a first electrode, a switching oxide layer comprising at least one transition metal oxide; and fabricating a second electrode on the switching oxide layer. In some embodiments, fabricating the second electrode includes fabricating a first layer comprising a first metallic material; and fabricating, on the first layer comprising the first metallic material, a second layer comprising a second metallic material.

In some embodiments, fabricating the first layer including the first metallic material includes fabricating a layer of titanium.

In some embodiments, fabricating the second layer of the second metallic material includes fabricating a layer of tantalum.

In some embodiments, fabricating the first layer comprising the first metallic material includes depositing a layer of a first metal having a thickness between 0.2 nm and 5 nm.

In some embodiments, fabricating the second layer including the second metallic material includes depositing a layer of a second metal that is thicker than the first layer comprising the first metallic material.

In some embodiments, the transition metal oxide includes at least one of HfO_(x) or TaO_(y), wherein x≤2.0, and wherein y≤2.5.

In some embodiments, the first electrode includes platinum or palladium.

In some embodiments, fabricating, on the first layer including the first metallic material, the second layer comprising the second metallic material includes fabricating an alloy of tantalum.

In some embodiments, the alloy of tantalum further includes at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.

In some embodiments, the alloy of tantalum includes at least one of a binary alloy comprising tantalum, a ternary alloy comprising tantalum, a quaternary alloy comprising tantalum, a quinary alloy comprising tantalum, a senary alloy comprising tantalum, or a high order alloy comprising tantalum.

According to one or more aspects of the present disclosure, an RRAM device may include a first electrode; a second electrode including an alloy of tantalum; and a switching oxide layer positioned between the first electrode and the second electrode. The switching oxide layer includes at least one transition metal oxide. The Ta alloy in the second electrode can have multiple elements, from binary alloy, ternary alloy, quaternary alloy, quinary alloy, senary alloy, and/or high order alloys.

In some embodiments, the alloy of tantalum further includes at least one of hafnium (Hf), molybdenum (Mo), niobium (Nb), tungsten (W), or zirconium (Zr).

In some embodiments, the RRAIVI further includes a substrate, wherein the second electrode is fabricated on the substrate, and wherein the switching oxide layer is fabricated on the second electrode.

According to one or more aspects of the present disclosure, a resistive random-access memory (RRAIVI) device is provided. The RRAM device may include: a first electrode; a second electrode including a first alloy containing tantalum; and a switching oxide layer positioned between the first electrode and the second electrode, wherein the switching oxide layer includes at least one transition metal oxide.

In some embodiments, the first alloy containing tantalum further includes at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.

In some embodiments, the first alloy containing tantalum includes at least one of a binary alloy containing tantalum, a ternary alloy containing tantalum, a quaternary alloy containing tantalum, a quinary alloy containing tantalum, a senary alloy containing tantalum, or a high order alloy containing tantalum.

In some embodiments, the second electrode further includes a second alloy containing tantalum.

In some embodiments, the second alloy containing tantalum further includes at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.

In some embodiments, the first alloy containing tantalum includes a binary alloy containing tantalum. In some embodiments, the second alloy containing tantalum includes a ternary alloy containing tantalum.

In some embodiments, the first alloy containing tantalum includes a first binary alloy containing tantalum, and wherein the second alloy containing tantalum includes a second binary alloy containing tantalum.

In some embodiments, the transition metal oxide includes at least one of HfO_(x) or TaO_(y), wherein x≤2.0, and wherein y≤2.5.

In some embodiments, the first electrode includes platinum and/or palladium.

In some embodiments, a dimension of the RRAM devices is between 1 μm and 1 nm.

In some embodiments, a dimension of the RRAM device is not greater than 0.28 μm.

According to one or more aspects of the present disclosure, a method for fabricating a resistive random-access memory (RRAM) device is provided. The method includes: fabricating, on a first electrode, a switching oxide layer including at least one transition metal oxide; and fabricating, on the switching oxide layer, a second electrode including a first alloy containing tantalum.

In some embodiments, the first alloy containing tantalum further includes at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.

In some embodiments, the first alloy containing tantalum includes at least one of a binary alloy containing tantalum, a ternary alloy containing tantalum, a quaternary alloy containing tantalum, a quinary alloy containing tantalum, a senary alloy containing tantalum, or a high order alloy containing tantalum.

In some embodiments, fabricating the second electrode further includes fabricating a second alloy containing tantalum.

In some embodiments, the first alloy containing tantalum includes a binary alloy containing tantalum, and wherein the second alloy containing tantalum includes a ternary alloy containing tantalum.

In some embodiments, the first alloy containing tantalum includes a first binary alloy containing tantalum, and wherein the second alloy containing tantalum includes a second binary alloy containing tantalum.

In some embodiments, the transition metal oxide includes at least one of HfO_(x) or TaO_(y), wherein x≤2.0, and wherein y≤2.5.

In some embodiments, the method further includes fabricating the first electrode on a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure. The drawings, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding.

FIG. 1 is a schematic diagram illustrating an example of a crossbar circuit in accordance with some implementations of the disclosure.

FIG. 2 is a schematic diagram illustrating an example of a cross-point device in accordance with some implementations of the disclosure.

FIGS. 3A, 3B, 3C, 4A, 4B, 5A, and 5B illustrate cross-sectional views of example RRAM devices in accordance with some embodiments of the present disclosure.

FIG. 6 is a flow diagram illustrating a method for fabricating an RRAM device in accordance with some embodiments of the present disclosure.

FIG. 7 is a flow diagram illustrating a method for fabricating a top electrode of an RRAM device in accordance with some embodiments of the present disclosure.

FIG. 8 is an I-V (current-voltage characteristic) curve of an RRAM device in accordance with some embodiments of the present disclosure.

FIG. 9 is an I-V curve showing the analog behavior of an RRAM device in accordance with some embodiments of the present disclosure.

FIG. 10 is a diagram showing the conductance characteristics of an RRAM device in accordance with some embodiments of the present disclosure.

FIG. 11 depicts a tantalum-titanium (Ta—Ti) binary phase diagram in accordance with some embodiments of the present disclosure.

FIG. 12A depicts a tantalum-hafnium (Ta—Hf) binary phase diagram in accordance with some embodiments of the present disclosure.

FIG. 12B depicts a tantalum-tungsten (Ta—W) binary phase diagram in accordance with some embodiments of the present disclosure.

FIG. 12C depicts a tantalum-molybdenum (Ta—Mo) binary phase diagram in accordance with some embodiments of the present disclosure.

FIG. 12D depicts a tantalum-niobium (Ta—Nb) binary phase diagram in accordance with some embodiments of the present disclosure.

FIG. 12E depicts a tantalum-zirconium (Ta—Zr) binary phase diagram in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

Aspects of the disclosure provide resistive random-access memory (RRAM) devices and methods for fabricating the RRAM devices. An RRAM device is a two-terminal passive device with tunable resistance. The RRAM device may include a first electrode, a second electrode, and a switching oxide layer positioned between the first electrode and the second electrode. In some embodiments, the first electrode and the second electrode may be a bottom electrode and a top electrode of the RRAM device, respectively. In some embodiments, the first electrode and the second electrode may be a top electrode and a bottom electrode of the RRAM device, respectively. The first electrode may include a nonreactive metal, such as platinum (Pt), palladium (Pd), etc. The second electrode may include a reactive metal, such as tantalum (Ta). The electrode including the nonreactive metal is also referred to herein as the “nonreactive electrode.” The electrode including the reactive metal is also referred to herein as the “reactive electrode.” The switching oxide layer may include a transition metal oxide, such as hafnium oxide (HfO_(x)) or tantalum oxide (TaO_(x)). The RRAM device may be in an initial state or virgin state and may have an initial high resistance before it is subject to any electrical simulation (e.g., a voltage or current signal applied to the RRAM device). The RRAM device may be tuned to a lower resistance state from the virgin state via a forming process or from a high-resistance state (HRS) to a lower resistance state (LRS) via a setting process. The forming process may refer to programming a device starting from the virgin state. The setting process may refer to programming a device starting from the high resistance state (HRS). After the reactive metal electrode being deposited on the switching oxide, the reactive metal can absorb oxygen from the switching oxide layer and create oxygen vacancies in the switching oxide layer, and oxygen ions can migrate in the switching oxide through a vacancy mechanism. During a forming process, a suitable programming signal (e.g., a voltage or current signal) may be applied to the RRAM device, which may cause a drift of oxygen ions to migrate from the switching oxide to the reactive electrode. As a result, a conductive channel or filament may form through the switching oxide layer (e.g., from the reactive electrode to the non-reactive electrode). The RRAM device may then be reset to a high-resistance state by applying a reset signal (e.g., a voltage signal, a current signal) to the RRAM device. The application of the reset signal to the RRAM device may cause oxygen to migrate back to the switching oxide layer and may thus interrupt the conductive filament. The RRAM device may be electrically switched between a high-resistance state and a low-resistance state by applying suitable programming signals (e.g., voltage signals, current signals, etc.) to the RRAM device. In a crossbar array circuit, the programming signals may be provided to the designated RRAM device via a selector, such as a transistor.

It might be desirable to scale down RRAM devices to a suitable size (e.g., a critical dimension of 100 nm, 10 nm, or a smaller dimension) to implement certain in-memory computing (IMC) applications (e.g., an IMC application that requires high density RRAM devices and/or low-power consumption). However, when the critical dimension of a conventional RRAM device scales down, the filament formed in the conventional RRAM device may not scale down accordingly. For example, the size of the filament formed in the scaled-down RRAM device may not be scaled-down proportionally. As such, forming, setting, and/or resetting such a conventional scaled-down RRAM device may still require a relatively high current or voltage. This may also prevent the effective scaling down of the selector (e.g., a transistor) and/or the integrated circuit that provides the current or voltage to the scaled-down RRAM device. Furthermore, the scaled-down RRAM device may have a relatively smaller area of top electrode. The top electrode may not be able to adsorb as much oxygen as that of a larger RRAM device. This may cause device failures and/or operation failures of the RRAM device. For example, a device failure can be caused by a delamination between the reactive electrode and the switching oxide by the presence of oxygen molecules. As another example, the oxygen ions may drift from the switching oxide into the top electrode under an external voltage and may migrate back to the switching oxide once the external voltage is removed, resulting in the operation to be volatile and an operation failure.

To address the aforementioned and other deficiencies of the conventional RRAM devices, the present disclosure provides RRAM devices with multi-component electrode structures that may enhance the performance of the RRAM devices and implement low-power IMC applications. In some embodiments, an RRAM device may include a bottom electrode, a top electrode, and a switching oxide layer positioned between the bottom electrode and the top electrode. The bottom electrode may include Pt or any other suitable nonactive metal. The switching oxide layer may include a transition metal oxide, such as HfO_(x), TaO_(x), TiO_(x), NbO_(x), ZrO_(x), etc.

The top electrode may include one or more multi-component electrode structures. For example, the top electrode may include one or more alloys of Ta in one implementation. The alloys of Ta may be and/or include a binary alloy containing Ta, a ternary alloy containing Ta, a quaternary alloy containing Ta, a quinary alloy containing Ta, a senary alloy containing Ta, and/or a high order alloy (e.g., an alloy containing more than six metallic elements) containing Ta. Each of the alloys may include Ta and one or more other metallic elements that have required thermodynamic and/or kinetic properties than Ta, such as tungsten (W), hafnium (Hf), molybdenum (Mo), niobium (Nb), zirconium (Zr), etc. For example, fabricating the top electrode using an alloy of Ta instead of pure Ta metal may reduce the migration of Ta into the switching oxide layer during the forming process and may thus reduce the size of the filament formed in the switching oxide layer (e.g., by reducing the lateral dimension or diameter of the filament). This may increase the filament resistance of the RRAM device and may thus increase the resistance of the RRAM device in both the low-resistance state and the high-resistance state, which may thus reduce the voltage and/or current required for operations of the RRAM device, such as forming, setting, resetting, and/or tuning the RRAM device. The RRAM device incorporating the multi-component electrode structure may present dynamic memristive behavior in multiple dimensions suitable for implementing dynamic learning, edge processing, inference engine accelerators, and other IMC applications.

In another implementation, the top electrode may include multiple layers of different metallic materials. For example, the top electrode may include a layer of titanium (Ti) and a layer of tantalum (Ta). The layer of Ti may be much thinner than the layer of Ta. For example, a thickness of the layer of Ti may be between about 0.2 nm and 5 nm. A thickness of the layer of Ta may be about 50 nm. In some embodiments, the thickness of the layer of Ti may be between 0.3 nm and 2 nm. Both Ti and Ta may trap and release oxygen during device operations. The incorporation of the thin Ti layer into the RRAM device may change the virgin resistance of the RRAM device, result in a less abrupt forming process, reduce the forming voltage, reduce the reset current, and reduce voltage and/or current requirements in subsequent operation processes.

Accordingly, the present disclosure provides techniques for fabricating RRAM devices with high filament resistance and reduced operation voltages and currents. The RRAM devices also present desirable linearity, analog, retention, and endurance etc. behaviors for IMC applications. The techniques may enable efficient scaling down of RRAM devices and low-power consumption IMC applications utilizing RRAM devices.

FIG. 1 is a schematic diagram illustrating an example 100 of a crossbar circuit in accordance some embodiments of the present disclosure. As shown, crossbar circuit 100 may include a plurality of interconnecting electrically conductive wires, such as one or more row wires 111 a, 111 b, . . . , 111 i, . . . , 111 n, and column wires 113 a, 113 b, . . . , 113 j, . . . , 113 m for an n-row by m-column crossbar array. The crossbar circuit 100 may further include cross-point devices 120 a, 120 b, . . . , 120 z, etc. Each of the cross-point devices may connect a row wire and a column wire. For example, the cross-point device 120 ij may connect the row wire 111 i and the column wire 113 j. In some embodiments, crossbar circuit 100 may further include digital to analog converters (DAC, not shown), analog to digital converters (ADC, not shown), switches (not shown), and/or any other suitable circuit components for implementing a crossbar-based apparatus. The number of the column wires 113 a-m and the number of the row wires 111 a-n may or may not be the same.

Row wires 111 may include a first row wire 111 a, a second row wire 111 b, . . . , 111 i, . . . , and a n-th row wire 111 n. Each of row wires 111 a, . . . , 111 n may be and/or include any suitable electrically conductive material. In some embodiments, each row wire 111 a-n may be a metal wire.

Column wires 113 may include a first column wire 113 a, a second column wire 113 b, . . . , and a m-th column wire 113 m. Each of column wires 113 a-m may be and/or include any suitable electrically conductive material. In some embodiments, each column wire 113 a-m may be a metal wire.

Each cross-point device 120 may be and/or include any suitable device with tunable resistance, such as a memristor, PCM devices, floating gates, spintronic devices, RRAM, SRAM, etc. In some embodiments, one or more of cross-point devices 120 may include an RRAM device as described in connection with FIGS. 3A-5B.

Crossbar circuit 100 may perform parallel weighted voltage multiplication and current summation. For example, an input voltage signal may be applied to one or more rows of crossbar circuit 100 (e.g., one or more selected rows). The input signal may flow through the cross-point devices of the rows of the crossbar circuit 100. The conductance of the cross-point device may be tuned to a specific value (also referred to as a “weight”). By Ohm's law, the input voltage multiplies the cross-point conductance generates a current form the cross-point device. By Kirchhoff's law, the summation of the current passing the devices on each column generates the current as the output signal, which may be read from the columns (e.g., outputs of the ADCs). According to Ohm's law and Kirchhoff s current law, the input-output relationship of the crossbar array can be represented as I=VG, wherein I represents the output signal matrix as current; V represents the input signal matrix as voltage; and G represents the conductance matrix of the cross-point devices. As such, the input signal is weighted at each of the cross-point devices by its conductance according to Ohm's law. The weighted current is outputted via each column wire and may be accumulated according to Kirchhoff's current law. This may enable in-memory computing (IMC) via parallel multiplications and summations performed in the crossbar arrays.

FIG. 2 is a schematic diagram illustrating an example 200 of a cross-point device in accordance some embodiments of the present disclosure. As shown, cross-point device 200 may connect a bitline (BL) 211, a select line (SEL) 213, and a wordline (WL) 215. The bitline 211 and the wordline 215 may be a column wire and a row wire as described in connection with FIG. 1, respectively.

Cross-point device 200 may include an RRAM device 201 and a transistor 203. A transistor is a three-terminal device, which may be marked as gate (G), source (S), and drain (D), respectively. The transistor 203 may be serially connected to RRAM device 201. As shown in FIG. 2, the first electrode of the RRAM device 201 may be connected to the drain of transistor 203. The second electrode of the RRAM device 201 may be connected to the bitline 211. The source of the transistor 203 may be connected to the wordline 215. The gate of the transistor 203 may be connected to the select line 213. RRAM device 201 may include one or more RRAM devices as described in connection with FIGS. 3A-5B below. Cross-point device 200 may also be referred to as in a 1-transitor-1-resistor (1T1R) configuration. The transistor 203 may perform as a selector as well as a current controller, which may set the current compliance, to the RRAM device 201 during programing. The gate voltage on transistor 203 can set current compliances to cross-point device 200 during programming and can thus control the conductance and analog behavior of cross-point device 200. For example, when cross-point device 200 is set from a high-resistance state to a low-resistance state, a set signal (e.g., a voltage signal, a current signal) may be provided via the bitline (BL) 211. Another voltage, also referred as a select voltage or gate voltage, may be applied via the select line (SEL) 213 to the transistor gate to open the gate and set the current compliance, while the wordline (WL) 215 may be set to ground. When cross-point device 200 is reset from the low-resistance state to the high-resistance state, a gate voltage may be applied to the gate of the transistor 203 via the select line 213 to open the transistor gate. Meanwhile, a reset signal may be sent to the RRAM device 201 via the wordline 215, while the bitline 211 may be set to ground.

FIGS. 3A, 3B, and 3C illustrate cross-sectional views of example RRAM devices in accordance with some embodiments of the present disclosure. RRAM devices 300 a, 300 b, and 300 c may correspond to an RRAM device in an initial state, a low-resistance state, and a high-resistance state, respectively.

As shown in FIG. 3A, RRAM device 300 a may include a substrate 310, a first electrode 320, a switching oxide layer 330, and a second electrode 340. RRAM device 300 a may further include one or more other components for implementing in-memory computing applications.

Substrate 310 may include one or more layers of any suitable material that may serve as a substrate for an RRAM device, such as silicon (Si), silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), aluminum nitride (AlN), etc. In some embodiments, substrate 310 may include diodes, transistors, interconnects, integrated circuits, etc. In some embodiments, the substrate may include a driving circuit including one or more electrical circuits (e.g., an array of electrical circuits) that may be individually controllable. In some embodiments, the driving circuit may include one or more complementary metal-oxide-semiconductor (CMOS) drivers.

First electrode 320 may be and/or include any suitable material that is electronically conductive and non-reactive to the switching oxide. For example, first electrode 320 may include platinum (Pt), palladium (Pd), iridium (Ir), titanium nitride (TiN), tantalum nitride (TaN), etc.

Switching oxide layer 330 may include one or more transition metal oxides, such as TaO_(x), HfO_(x), TiO_(x), NbOx, ZrOx, etc., in binary oxides, ternary oxides, and high order oxides. In some embodiments, the chemical stability of the non-reactive material in first electrode 320 may be higher than that of the transition metal oxide(s) in switching oxide layer 330.

Second electrode 340 may include any suitable metallic material that are electronically conductive and reactive to the switching oxide. For example, the metallic material in second electrode 340 may include Ta, Hf, Ti, TiN, TaN, etc. Second electrode 340 may be reactive to the switching oxide and may have suitable oxygen solubility to adsorb some oxygen from the switching oxide and create oxygen vacancies in the switching oxide. In other words, the reactive metallic material(s) in second electrode 340 may have suitable oxygen solubility and/or oxygen mobility. In some embodiments, second electrode 340 may not only be able to create oxygen vacancies in switching oxide layer 330 (e.g., by scavenging oxygen), but may also function as oxygen reservoir or source to the switching oxide layer 330 during cell programming.

RRAM device 300 a may have an initial resistance (also referred to herein as the “virgin resistance”) after it is fabricated. The initial resistance of RRAM device 300 a may be changed and RRAM device 300 a may be switched to a state of a lower resistance via a forming process. For example, a suitable voltage or current may be applied to RRAM device 300 a. The application of the voltage to RRAM device 300 a may induce the metallic material(s) in the second electrode to absorb oxygen from the switching oxide layer 330 and create oxygen vacancies in the switching oxide layer 330. As a result, a conductive channel (e.g., a filament) which is oxygen vacancy rich may form in the switching oxide layer 330. For example, as illustrated in FIG. 3B, a conductive channel 335 a may be formed in the switching oxide layer 330. As shown, conductive channel 335 a may be formed from the second electrode 340 to the first electrode 320 across the switching oxide layer 330. RRAM device 300 b may be reset to a high-resistance state. For example, a reset signal (e.g., a voltage signal or a current signal) may be applied to RRAM device 300 b during a reset process. In some embodiments, the set signal and the reset signal may have opposite polarity, i.e., a positive signal and a negative signal, respectively. The application of the reset signal may cause oxygen to drift back to the switching oxide layer 330 and recombine with one or more of the oxygen vacancies. For example, an interrupted conductive channel 335 b as shown in FIG. 3C may be formed in the switching oxide layer 330 during the reset process. As shown, the conductive channel may be interrupted with a gap between the interrupted conductive channel 335 b and the first electrode 320. The lateral dimension of conductive channel 335 b may be smaller than that of the conductive channel 335 a. In some embodiments, conductive channel 335 b does not continuously connect the first electrode 320 and the second electrode 340. RRAM device 300 a-c may be electrically switched between the high-resistance state and the low-resistance state by applying suitable programming signals (e.g., voltage signals, current signals, etc.) to the RRAM device.

In one implementation, the second electrode 340 may include one or more alloys. Each of the alloys may contain two or more metallic elements. Each of the alloys may include a binary alloy (e.g., an alloy containing two metallic elements), a ternary alloy (e.g., an alloy containing three metallic elements), a quaternary alloy (e.g., an alloy containing four metallic elements), a quinary alloy (e.g., an alloy containing five metallic elements), a senary alloy (e.g., an alloy containing six metallic elements), and/or a high order alloy (e.g., an alloy containing more than six metallic elements). In some embodiments, the second electrode 340 may include one or more alloys containing a first metallic element and one or more second metallic elements. Each of the second metallic elements may be less or more reactive to the transition metal oxide in the switching oxide layer than the first metallic element. In some embodiments, the first metallic element may be Ta. The second metallic elements may include one or more of W, Hf, Mo, Nb, Zr, etc. In some embodiments, the ratio of the first metallic element to the second metallic element(s) in an alloy in the second electrode 340 may be about 50 atomic percent. In some embodiments, the suitable ratio of the first metallic element to the second metallic element in the alloy may be optimized from the entire composition range. During a forming process, the second metallic element(s) may create fewer oxygen vacancies in the switching oxide layer than the first metallic element. As such, the lateral size of the filament formed in an RRAM device comprising a second electrode containing the alloy may be smaller than that of the filament formed in an RRAM device comprising a second electrode made of only the first metal. For example, as illustrated in FIGS. 4A and 4B, each of RRAIVI devices 400 a and 400 b may include the first electrode 320 and the switching oxide layer 330 as described above in conjunction with FIGS. 3A-3C. RRAM device 400 a may include a second electrode 340 a that includes one or more layers of the first metallic element (e.g., Ta). RRAIVI device 400 b may include a second electrode 340 b that includes one or more alloys of the first metallic element (e.g., one or more binary alloys of Ta, ternary alloys containing Ta, quaternary alloys containing Ta, quinary alloys containing Ta, senary alloys containing Ta, high order alloys containing Ta, etc.). During a forming process, a filament 435 a and a filament 435 b may be formed in RRAM devices 400 a and 400 b, respectively. As illustrated, a relatively strong filament 435 a may connect first electrode 320 and second electrode 340 a for a high conductance. A relatively weak filament 435 b may connect first electrode 320 and second electrode 340 b for a low conductance. The lateral dimension of filament 435 b may be smaller than that of the filament 435 a. As such, the filament resistance of RRAM device 400 b may be higher than that of RRAM device 400 a and the resistance of RRAM device 400 b may thus be higher than that of RRAM device 400 a, resulting in low current operations for IMC applications.

As an example, the second electrode 340 b may include one or more alloys containing Ta (also referred to as “Ta alloys”). Each of the Ta alloys may include Ta and one or more other metallic elements (e.g., Hf, W, Mo, Nb, Zr, etc.). As an example, the second electrode 340 b may include one or more binary alloys containing Ta. Examples of the binary alloys containing Ta include a Ta—Hf alloy, a Ta—W alloy, a Ta—Mo alloy, a Ta—Nb alloy, a Ta—Zr alloy, etc. As another example, the second electrode 340 b may include one or more ternary alloys containing Ta. Examples of the ternary alloys containing Ta include a Ta—Hf—Mo alloy, a Ta—Hf—Nb alloy, a Ta—Hf—W alloy, a Ta—Hf—Zr alloy, a Ta—Mo—Nb alloy, a Ta—Mo—W alloy, a Ta—Mo—Zr alloy, a Ta—Nb—W alloy, a Ta—Nb—Zr alloy, a Ta—W—Zr alloy, etc. As still another example, the second electrode 340 b may include one or more quaternary alloys containing Ta. Examples of the quaternary alloys containing Ta include a Ta—Hf—Mo—Nb alloy, a Ta—Hf—Mo—W alloy, a Ta—Hf—Mo—Zr alloy, a Ta—Hf—Nb—W alloy, a Ta—Hf—Nb—Zr alloy, a Ta—Mo—Nb—W alloy, a Ta—Mo—Nb—Zr alloy, a Ta—Nb—W—Zr alloy, a Ta—Mo—Nb—W alloy, a Ta—Mo—Nb—Zr alloy, etc. As a further example, the second electrode 340 b may include one or more quinary alloys containing Ta. Examples of the quinary alloys containing Ta include a Ta—Hf—Mo—Nb—W alloy, a Ta—Mo—Nb—W—Zr alloy, a Ta—Hf—Nb—W—Zr alloy, a Ta—Hf—Mo—W—Zr alloy, a Ta—Hf—Mo—Nb—Zr alloy, etc. As still a further example, the second electrode 340 b may include a senary alloy containing Ta, such as a Ta—Hf—Mo—Nb—W—Zr alloy. As a further example, the second electrode 340 b may include a high order alloy containing Ta. In some embodiments, the high order alloy may further contain vanadium (V).

In some embodiments, the second electrode 340 b may include a plurality of alloys. Each of the alloys may be a Ta alloy containing Ta and one or more other metallic elements (e.g., Hf, W, Mo, Nb, Zr, etc.). The Ta alloy may be and/or include a binary alloy, a ternary alloy, a quaternary alloy, a quinary alloy, a senary alloy, a high-order alloy, etc.

As an example, the second electrode 340 b may include two or more of a first alloy containing Ta, a second alloy containing Ta, a third alloy containing Ta, a fourth alloy containing Ta, a fifth alloy containing Ta, and a sixth alloy containing Ta. In some embodiments, the first alloy containing Ta, the second alloy containing Ta, the third alloy containing Ta, the fourth alloy containing Ta, the fifth alloy containing Ta, and the sixth alloy containing Ta may be a binary alloy, a ternary alloy, a quaternary alloy, a quinary alloy, a senary alloy, and a high-order alloy, respectively.

In some embodiments, multiple alloys in the second electrode 340 b may correspond to combinations of the same number of metallic elements. For example, the first alloy containing Ta and the second alloy containing Ta may include a first binary alloy containing Ta (e.g., a Ta—W alloy) and a second binary alloy containing Ta (e.g., a Ta—Mo alloy), respectively. As another example, the first alloy containing Ta and the second alloy containing Ta may include a first ternary alloy containing Ta (e.g., a Ta—Hf—Mo alloy) and a second ternary alloy containing Ta (e.g., a Ta—Hf—Nb alloy), respectively. As a further example, the second electrode 340 b may include a plurality of alloy systems. Each of the alloy systems may contain the alloys containing the mixtures of certain metallic elements with varying compositions. For example, a binary system may include one or more binary alloys of two metallic elements (e.g., Ta and Hf) with varying compositions. Each of the binary alloys may be a combination of the two metallic elements with a certain composition. As another example, a ternary system may include one or more ternary alloys of three metallic elements (e.g., Ta, Hf, and W) with varying compositions. Each of the ternary alloys may be a combination of the three metallic elements with a certain composition. In some embodiments, the second electrode 340 b may include a Ta alloy system containing one or more alloy systems. In some embodiments, the Ta alloy system may include two or more alloy systems. For example, the Ta alloy system may contain a senary system containing one or more alloys of Ta, Hf, W, Mo, Nb, and Zr. The Ta alloy system may further include one or more binary systems, ternary systems, quaternary systems, and/or quinary systems containing Ta alloys. The binary systems may include one or more of a Ta—Hf alloy system, a Ta—W alloy system, a Ta—Mo alloy system, a Ta—Nb alloy system, and/or a Ta—Zr alloy system. The ternary systems may include one or more of a Ta—Hf—Mo alloy system, a Ta—Hf—Nb alloy system, a Ta—Hf—W alloy system, a Ta—Hf—Zr alloy system, a Ta—Mo—Nb alloy system, a Ta—Mo—W alloy system, a Ta—Mo—Zr alloy system, a Ta—Nb—W alloy system, a Ta—Nb—Zr alloy system, and/or a Ta—W—Zr alloy system. The quaternary systems may include one or more of a Ta—Hf—Mo—Nb alloy system, a Ta—Hf—Mo—W alloy system, a Ta—Hf—Mo—Zr alloy system, a Ta—Hf—Nb—W alloy system, a Ta—Hf—Nb—Zr alloy system, a Ta—Mo—Nb—W alloy system, a Ta—Mo—Nb—Zr alloy system, a Ta—Nb—W—Zr alloy system, a Ta—Mo—Nb—W alloy system, and a Ta—Mo—Nb—Zr alloy system. The quinary systems may include one or more of a Ta—Hf—Mo—Nb—W alloy system, a Ta—Mo—Nb—W—Zr alloy system, a Ta—Hf—Nb—W—Zr alloy system, a Ta—Hf—Mo—W—Zr alloy system, and/or a Ta—Hf—Mo—Nb—Zr alloy system. Each of the alloy systems contained in the second electrode 340 b may have unique thermodynamic and kinetic characteristics and may be regarded as an electrode component. As such, the second electrode 340 b may include multiple electrode components for providing multiple state variables that may lead to rich dynamics with various time constants for computing and learning. For example, each electrode component may have different reactivity to the switching oxide or the affinity for oxygen. Each electrode element may have different diffusivity (e.g., self-diffusion, inter-diffusion, diffusion time constants, etc.). The second electrode 340 b with multiple components may provide the multiple dynamic behavior for IMC applications. As such, the RRAM device incorporating the multi-component second electrode may present dynamic memristive behavior in multiple dimensions.

FIGS. 5A and 5B are schematic diagrams illustrating cross-section views of an RRAM device in accordance with some embodiments of the present disclosure. RRAM devices 500 a and 500 b may correspond to an initial state and a low-resistance state of the RRAM device, respectively.

As shown, RRAM device 500 a may include a substrate 310, a first electrode 320, a switching oxide layer 330, and a second electrode 340 c. The substrate 310, the first electrode 320, and the switching oxide layer 330 may be the same as their counterparts as described in connection with FIGS. 3A-3C. The second electrode 340 c may include a first layer 341 and a second layer 343. The first layer 341 may include a first metallic material that may scavenge oxygen from the transition metal oxide in the switching oxide layer. The second layer 343 may include a second metallic material that may scavenge oxygen from the transition metal oxide in the switching oxide layer. The first metallic material and the second metallic material may include different chemical elements and may have different affinities for oxygen and/or different thermodynamic and kinetic properties. The first metallic material and the second metallic material may be immiscible. In some embodiments, the first metallic material may include Ti. The second metallic material may include Ta. In some embodiments, the first layer 341 may be and/or include a layer of Ti metal (e.g., a Ti film). The second layer 343 may be and/or include a layer of Ta metal (e.g., a Ta film). In some embodiments, the second layer 343 may be and/or include one or more alloys of Ta, such as one or more binary alloys of Ta, ternary alloys containing Ta, quaternary alloys containing Ta, quinary alloys containing Ta, senary alloys containing Ta, high order alloys containing Ta, etc. as described in connection with FIG. 4B.

As shown in the Ta—Ti binary phase diagram of FIG. 11, Ta and Ti are immiscible at operating temperatures of RRAM devices (e.g., temperatures around or above room temperature). As such, the addition of Ti into the RRAM device may not affect the operation mechanism of the Ta filament in the switching oxide and the switching mechanism of RRAM devices as described herein. The RRAM devices as described in connection with FIGS. 5A-B may thus be used for IMC applications that require RRAM devices with excellent performance in analog behaviors, linearity, retention, reliability, etc. The immiscibility between Ta and Ti may also enable a thermodynamic equilibrium between the second layer 343 and the first layer 341. As a result, a thin Ti film can function as designed without reacting with the Ta film or being dissolved by the Ta film.

Furthermore, Ti may readily scavenge oxygen from the switching oxide because it has higher affinity for oxygen than Ta. As such, the incorporation of the first layer 341 into the RRAM device may further improve the performance of the RRAM device by reducing the forming voltage required in the RRAM forming process and the current and voltage requirements in subsequent operations. For example, during a forming process, both the first metallic material and the second metallic material may generate oxygen vacancies in the switching oxide layer 330. For example, as shown in FIG. 5B, a filament 435 c may be formed in the switching oxide layer 330. As illustrated, filament 435 c may connect the first electrode 320 and the second electrode 340 c. The lateral dimension of the filament 435 c of FIG. 5B may be smaller than that of the filament 435 a of FIG. 4A. As such, the filament resistance of RRAM device 500 b may be higher than that of RRAM device 400 a. As Ti has a higher affinity for oxygen than Ta, the virgin resistance of RRAM device 500 b may be lower than that of RRAM device 400 a, resulting in the lower forming voltage, the lower reset voltage, the low reset current, etc.

Ti may also readily store oxygen during reset (when oxygen is migrating from switching oxide to the second electrode). This may enable the second electrode to store oxygen during the reset process and may thus prevent device failures (which can be caused by the presence of oxygen molecules between switching oxide and the electrode) and/or operation failures (which can be caused by the oxygen migrating back to the switching oxide once the reset voltage being remove, or the switch being volatile).

The first layer 341 may be grown to a suitable thickness so that the first metallic material (e.g., Ti) of the first layer 341 may function as described above without affecting the formation of the filament comprising the second metallic material (e.g., a Ta filament) in the switching oxide layer 330. In some embodiments, a thickness of the first layer 341 may be between about 0.2 nm and about 5 nm. In some embodiments, a thickness of the first layer 341 may be between about 0.3 nm and about 2 nm. In some embodiments, a thickness of the first layer 341 may be about 1 nm. In some embodiments, a thickness of the first layer 341 may be less than 1 nm. The second layer 343 may be thicker than the first layer 341. In some embodiments, a thickness of the second layer 343 may be between 5 nm and 300 nm. For example, the thickness of the second layer 343 may be between about 10 nm and about 100 nm. In some embodiments, the thickness of the second layer 343 may be between about 10 nm and about 200 nm. In some embodiments, the thickness of the second layer 343 may be about 50 nm. A thickness of the first electrode may be between about 5 nm and 100 nm. In some embodiments, the thickness of the first electrode may be about 30 nm. In some embodiments, a dimension (e.g., a critical dimension) of RRAM device 400 a, 400 b, 500 a, and/or 500 b may be between 1 μm and single digit nanometers. In some embodiments, the critical dimension of RRAM device 400 a, 400 b, 500 a, and/or 500 b may be about or less than 0.28 μm, and/or between 1 μm and 1 nanometer. In some embodiments, the critical dimension of RRAM device 400 a, 400 b, 500 a, and/or 500 b may be between 1 μm and 2 nm. In some embodiments, the critical dimension of RRAM device 400 a, 400 b, 500 a, and/or 500 b may be between 1 μm and 5 nm. In some embodiments, the critical dimension of RRAM device 400 a, 400 b, 500 a, and/or 500 b may be at the single-digit nanoscale (e.g., between about 1 nm and about 9 nm).

In one implementation, the second layer 343 may be fabricated directly on the first layer 341. For example, as shown in FIGS. 5A-5B, a surface of the second layer 343 may directly contact a surface of the first layer 341. In another implementation, one or more other layers of suitable materials may be deposited between the first layer 341 and the second layer 343.

The incorporation of the first layer 341 in RRAM device 500 a-b may reduce the virgin resistance of the RRAM device, reduce the forming voltage, and reduce the reset current, resulting in a less abrupt forming process, a filament with lower conductance, and lower voltage and current in the subsequent operation process.

While certain components of RRAM devices 300 a-c, 400 a-b, and 500 a-b are shown in FIGS. 3A-5B, this is merely illustrative. RRAM devices 300 a-c, 400 a-b, and 500 a-b may include one or more other layers of suitable materials for implementing IMC applications. For example, one or more interfacial layers (not shown) may be fabricated between the switching oxide layer and one or more of the second electrode and the first electrode to improve the interface stability and device performance.

FIG. 6 is a flow diagram illustrating an example 600 of a method for fabricating an RRAM device according to some embodiments of the disclosure.

At block 610, a first electrode may be fabricated on a substrate. Fabricating the first electrode may involve depositing one or more layers of one or more nonactive metals, such as Pt, Pd, Ir, etc. utilizing a physical vapor deposition (PVD) technique, a chemical vapor deposition (CVD) technique, a sputtering deposition technique, an atomic layer deposition (ALD) technique, and/or any other suitable deposition technique. In some embodiments, fabricating the first electrode may involve depositing one or more layers of Pt. The first electrode may be and/or include first electrode 320 as described in connection with FIGS. 3A-5B above.

At block 620, a switching oxide layer comprising one or more transition metal oxides may be fabricated on the first electrode. The transition metal oxides may include, for example, TaO_(x), HfO_(x), TiO_(x), NbO_(x), ZrO_(x), etc. The switching oxide layer may be deposited utilizing an atomic layer deposition (ALD) technique, physical vapor deposition (PVD) technique, chemical vapor deposition (CVD) technique, and/or any other suitable deposition technique. The switching oxide layer may be and/or include switching oxide layer 330 as described in connection with FIGS. 3A-5B above.

At block 630, a second electrode may be fabricated on the switching oxide layer. Fabricating the second electrode may involve fabricating one or more layers of one or more metallic materials that are electronically conductive and reactive to the switching oxide. In some embodiments, fabricating the second electrode may involve fabricating one or more alloys. Each of the alloys may contain a first metallic element and one or more second metallic elements. Each of the second metallic elements and the first metallic element may have different reactivity to the transition metal oxide in the switching oxide layer. In some embodiments, the first metallic element may be Ta. The second metallic elements may be one or more of W, Hf, Mo, Nb, Zr, etc. Based on the binary phase diagrams involving Ta and the second metallic elements W, Hf, Mo, Nb, or Zr as shown in FIGS. 12A-12E, Ta—W (FIG. 12B), Ta—Mo (FIG. 12C), and Ta—Nb (FIG. 12D) form continuous solid solution, and Ta—Hf (FIG. 12A) and Ta—Zr (FIG. 12E) are immiscible at RRAM devices operation temperatures. No binary intermetallic compounds may form in all these binaries. No intermetallic compounds in these binary systems are advantageous for IMC applications involving an alloy electrode which may be readily fabricated and controlled. For example, fabricating the second electrode may involve fabricating the alloy by co-sputtering the first metal and the second metal. As another example, fabricating the second electrode may involve sputtering from an alloy target (e.g., a Ta—W alloy, Ta—Hf alloy, Ta—Mo alloy, Ta—Nb alloy, Ta—Zr alloy, etc.) for a required composition between the first metallic element (e.g., a pure Ta metal) and the second metallic element(s) (e.g., a pure Hf metal). In some embodiments, fabricating the second electrode may involve fabricating multiple electrode components including Ta, Hf, Nb, Mo, W, and/or Zr. Each of the electrode components may be and/or include a binary alloy, a ternary alloy, a quaternary alloy, a quinary alloy, a senary alloy, and/or a high order alloy of Ta. For example, fabricating the second electrode may involve fabricating the second electrode 340 b with one or more alloys and/or alloy systems as described in connection with FIG. 4B. More particularly, for example, fabricating the second electrode may involve fabricating two or more of a first alloy containing Ta, a second alloy containing Ta, a third alloy containing Ta, a fourth alloy containing Ta, a fifth alloy containing Ta, and a sixth alloy containing Ta. The first alloy containing Ta, the second alloy containing Ta, the third alloy containing Ta, the fourth alloy containing Ta, the fifth alloy containing Ta, and the sixth alloy containing Ta may be a binary alloy, a ternary alloy, a quaternary alloy, a quinary alloy, a senary alloy, and a high-order alloy, respectively.

In some embodiments, fabricating the second electrode may involve fabricating multiple layers of multiple metallic materials, such as layers 341 and 343 as described in conjunction with FIGS. 5A-5B. In some embodiments, the second electrode may be fabricated by performing one or more operations as described in connection with FIG. 7.

FIG. 7 is a flow diagram illustrating an example 700 of a method for fabricating a second electrode of an RRAM device according to some embodiments of the disclosure.

At block 710, a first layer including a first metallic material may be fabricated. The first metallic material may include a first metallic element, such as Ti. The first layer of the first metallic material may be fabricated by depositing a first metal (e.g., Ti metal) utilizing PVD, CVD, sputtering, ALD, and/or any other suitable deposition technique. Fabricating the first layer of the first metal may involve depositing a layer of the first metal with a suitable thickness, such as a thickness between about 0.2 nm and about 5 nm, a thickness between about 0.3 nm and about 2 nm, etc.

At block 720, a second layer including a second metallic material may be fabricated. The second metallic material may include a second metallic element that is different from the first metallic element. For example, the second metallic element may be Ta. In some embodiments, fabricating the second layer including the second metallic material may involve depositing a second metal (e.g., Ta metal) utilizing PVD, CVD, sputtering, ALD, and/or any other suitable deposition technique. Fabricating the second layer of the second metal may involve depositing a layer of the second metal with a suitable thickness, such as a layer of the second metal that is thicker than that of the first layer of the first metal. In some embodiments, a layer of the second metal having a thickness between 10 nm and 100 nm may be deposited. In some embodiments, the second layer of the second metal may be deposited directly on the first layer of the first metal. In such embodiments, a surface of the first layer of the first metal may directly contact a surface of the second layer of the second metal.

In some embodiments, fabricating the second layer including the second metallic material may involve fabricating a layer including one or more alloys. As described above, each of the alloys may contain a first metallic element and one or more second metallic elements. Each of the second metallic elements may have different reactivity to the transition metal oxide in the switching oxide layer than the first metallic element. In some embodiments, the first metallic element may be Ta. The second metallic elements may be one or more of W, Hf, Mo, Nb, Zr, etc. Fabricating the second layer of the Ta alloy may involve depositing the Ta alloy utilizing PVD, CVD, sputtering, ALD, and/or any other suitable deposition technique. Fabricating the second layer of the Ta alloy may involve depositing a layer of the Ta alloy with a suitable thickness, such as a layer of the Ta alloy that is thicker than that of the first layer of the first metal. In some embodiments, a layer of one or more Ta alloys having a thickness between about 5 nm and about 100 nm may be deposited.

FIG. 8 is an I-V (current-voltage) characterization of an RRAM device in accordance with some embodiments of the present disclosure in FIGS. 5A-5B. FIG. 9 is an I-V curve showing the analog behavior of the RRAM device described in connection with FIGS. 5A-5B. FIG. 10 is a diagram showing the conductance characteristics of the RRAM device described in connection with FIGS. 5A-5B. In this example, the dimensions of the RRAM device may be about 0.24 μm. The RRAM device may include the second electrode 340 c, the switching oxide layer 330, and the first electrode as described in connection with FIGS. 5A-5B. The second electrode 340C may include the first layer 341 of Ti having a thickness of about 1 nm and the second layer 343 of Ta having a thickness of about 50 nm. The first electrode 320 may include Pt. The switching oxide layer 330 may include HfO_(x).

As shown in FIG. 8, the RRAM device present repeatable and desirable set-reset operations.

As shown in FIG. 9, the RRAM device present desirable analog behavior, that the device resistance can be tuned to multi-levels (or analog behavior) by controlling the current compliance, and the current is linearly proportional to the voltage (or linearity behavior) at each resistance state.

As shown in FIG. 10, the RRAM device present desirable resistance state stability. The RRAM device was turned to various levels of conductance and showed excellent stability with time at each conductance (or the retention of the resistance states).

For simplicity of explanation, the methods of this disclosure are depicted and described as a series of acts. However, acts in accordance with this disclosure can occur in various orders and/or concurrently, and with other acts not presented and described herein. Furthermore, not all illustrated acts may be required to implement the methods in accordance with the disclosed subject matter. In addition, those skilled in the art will understand and appreciate that the methods could alternatively be represented as a series of interrelated states via a state diagram or events.

The terms “approximately,” “about,” and “substantially” as used herein may mean within a range of normal tolerance in the art, such as within 2 standard deviations of the mean, within ±20% of a target dimension in some embodiments, within ±10% of a target dimension in some embodiments, within ±5% of a target dimension in some embodiments, within ±2% of a target dimension in some embodiments, within ±1% of a target dimension in some embodiments, and yet within ±0.1% of a target dimension in some embodiments. The terms “approximately” and “about” may include the target dimension. Unless specifically stated or obvious from context, all numerical values described herein are modified by the term “about.”

As used herein, a range includes all the values within the range. For example, a range of 1 to 10 may include any number, combination of numbers, sub-range from the numbers of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 and fractions thereof.

In the foregoing description, numerous details are set forth. It will be apparent, however, that the disclosure may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the disclosure.

The terms “first,” “second,” “third,” “fourth,” etc. as used herein are meant as labels to distinguish among different elements and may not necessarily have an ordinal meaning according to their numerical designation.

The words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X includes A or B” is intended to mean any of the natural inclusive permutations. That is, if X includes A; X includes B; or X includes both A and B, then “X includes A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Reference throughout this specification to “an implementation” or “one implementation” means that a particular feature, structure, or characteristic described in connection with the implementation is included in at least one implementation. Thus, the appearances of the phrase “an implementation” or “one implementation” in various places throughout this specification are not necessarily all referring to the same implementation.

As used herein, when an element or layer is referred to as being “on” another element or layer, the element or layer may be directly on the other element or layer, or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on” another element or layer, there are no intervening elements or layers present.

Whereas many alterations and modifications of the disclosure will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims, which in themselves recite only those features regarded as the disclosure. 

What is claimed is:
 1. A resistive random-access memory (RRAM) device, comprising: a first electrode; a second electrode comprising: a first layer comprising a first metallic material; and a second layer comprising a second metallic material, wherein the second layer comprising the second metallic material is fabricated on the first layer comprising the first metallic material; and a switching oxide layer positioned between the first electrode and the second electrode, wherein the switching oxide layer comprises at least one transition metal oxide.
 2. The RRAM device of claim 1, wherein the first metallic material in the second electrode comprises titanium.
 3. The RRAM device of claim 1, wherein the second metallic material in the second electrode comprises tantalum.
 4. The RRAM device of claim 1, wherein a thickness of the first layer comprising the first metallic material is between 0.2 nm and 5 nm.
 5. The RRAM device of claim 4, wherein a thickness of the second layer comprising the second metallic material is between 5 nm and 300 nm.
 6. The RRAM device of claim 1, wherein the at least one transition metal oxide comprises at least one of HfO_(x) or TaO_(y), wherein x≤2.0, and wherein y≤2.5.
 7. The RRAM device of claim 1, wherein the first electrode comprises at least one of platinum or palladium.
 8. The RRAM device of claim 1, wherein a dimension of the RRAM device is not greater than 1 micrometer (μm), and wherein a dimension of the RRAM device is between 1 μm and 1 nm.
 9. The RRAM device of claim 1, wherein the second layer in the second electrode comprises an alloy of tantalum.
 10. The RRAM device of claim 9, wherein the alloy of tantalum further comprises at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.
 11. The RRAM device of claim 10, wherein the alloy of tantalum comprises at least one of a binary alloy comprising tantalum, a ternary alloy comprising tantalum, a quaternary alloy comprising tantalum, a quinary alloy comprising tantalum, a senary alloy comprising tantalum, or a high order alloy comprising tantalum.
 12. A method for fabricating a resistive random-access memory (RRAM) device, comprising: fabricating, on a first electrode, a switching oxide layer comprising at least one transition metal oxide; and fabricating a second electrode on the switching oxide layer, comprising: fabricating a first layer comprising a first metallic material; and fabricating, on the first layer comprising the first metallic material, a second layer comprising a second metallic material.
 13. The method of claim 12, wherein fabricating the first layer comprising the first metallic material comprises fabricating a layer of titanium.
 14. The method of claim 13, wherein fabricating the second layer of the second metallic material comprises fabricating a layer of tantalum.
 15. The method of claim 14, wherein fabricating the first layer comprising the first metallic material comprises depositing a layer of a first metal having a thickness between 0.2 nm and 5 nm.
 16. The method of claim 14, wherein the at least one transition metal oxide comprises at least one of HfO_(x) or TaO_(y), wherein x≤2.0, and wherein y≤2.5.
 17. The method of claim 14, wherein the first electrode comprises platinum or palladium.
 18. The method of claim 14, wherein fabricating, on the first layer comprising the first metallic material, the second layer comprising the second metallic material comprises fabricating an alloy of tantalum.
 19. The method of claim 18, wherein the alloy of tantalum further comprises at least one of hafnium, molybdenum, tungsten, niobium, or zirconium.
 20. The method of claim 19, wherein the alloy of tantalum comprises at least one of a binary alloy comprising tantalum, a ternary alloy comprising tantalum, a quaternary alloy comprising tantalum, a quinary alloy comprising tantalum, a senary alloy comprising tantalum, or a high order alloy comprising tantalum. 